1. Field of the Invention
This invention relates, in general, to an apparatus for measuring voltage and more particularly to apparatus for measuring the true root-mean-square (rms) voltage of an applied voltage signal.
2. Background of the Invention
True rms voltage electronic measurement devices are known and widely used. These devices electronically convert the AC voltage to a direct current (DC) output by squaring the voltage, averaging and then obtaining a square root. Integrated circuits (ICs) such as the AD536 from Analog Devices, Norwood, Mass. have less than 1% error at frequencies up to about 140 kHz with a 7 V rms input and 6 kHz at a 10 mV rms input. A wide-bandwidth multiplier (squarer) such as the AD834 allows input bandwidth from 5 Hz to over 20 MHz and a peak input of 10V. The dynamic range of such devices is limited because the squarer must deal with a signal that varies enormously in amplitude. For example, an input signal of 1 mV to 100 mV results in a 1 mV to 10,000 mV (10V) at the output of the squarer. Because of this effect, such devices are typically limited to a 10:1 dynamic range. To overcome this difficulty, the average of the output of the circuit is used to divide the input of the circuit. As such, the signals vary linearly rather than as the square of the input voltage. Although this increases the dynamic range of the circuit, it comes at the expense of less bandwidth.
For the most accurate true rms voltage measurement, thermal voltage converter devices are used. These devices measure the rms value of the voltage by applying the unknown voltage to a heating element and then measuring the temperature change produced in the heating element. By comparing the heating value of an unknown ac signal to the heating value of a known calibrated dc reference, the value of the dc reference will equal the rms value of the unknown signal. Instruments such as the Fluke 540, WaveTek/Datron 4920M, and other thermal voltage converters provide excellent performance at frequencies up to 1 MHz where the error is less than 0.1%, i.e., 100 ppm. Above 1 MHz, the error is about 1% while at 20 MHz the error increases to about 2%. Although the accuracy of the thermal voltage converter is superior to integrated circuit (IC)-based devices, the instruments are very fragile, have a limited dynamic range (typically of the order of 10 db), and are easily damaged by small overloads. Moreover, the heating process is relatively slow and making a series of measurements at just one frequency is very time consuming.
In an effort to overcome some of the prior art limitations, Paulter (N. G. Paulter, xe2x80x9cAn electro-optic-based RMS voltage measurement technique,xe2x80x9d Rev. Sci. Instrum., Vol. 66, No. 6, Jun. 1995, pp. 3683-3690) has developed an electro-optic device. The Paulter approach is based on an electro-optic cell that requires bulk optic components such as a large crystal, light beam splitters, lenses, and polarizers which introduce their own set of problems including alignment and stabilization considerations. Such bulk components and supporting setup are neither light weight nor portable. Further, since the device operates as a square law device, the range of voltage that can be handled is severely limited.
In order to overcome these and other problems of the prior art instruments, it is an object of the present invention to provide a true root-mean square ac measuring device that utilizes an integrated electro-optical device.
It is another object of the present invention to provide a true root-mean square ac measuring device that has a high measurement bandwidth.
It is another object of the present invention to provide a true root-mean square ac measuring device that has a high damage overload threshold.
It is another object of the present invention to provide a true root-mean square ac measuring device that is compact in size.
It is another object of the present invention to provide a true root-mean square ac measuring device that has high sensitivity.
It is another object of the present invention to provide a true root-mean square ac measuring device that has high measurement reliability.
It is another object of the present invention to provide a true root-mean square ac measuring device that is optically isolated from its input source.
Yet another object of the present invention is to provide a true root-mean square ac measuring device that provides temperature stability to an electro-optical component.
It is another object of the present invention to provide a true root-mean square ac measuring device that provides null correction to an electro-optical component.
It is another object of the present invention to provide a true root-mean square ac measuring device that provides an ac reference voltage to an electro-optical component.
It is another object of the present invention to provide a true root-mean square ac measuring device that provides frequency correction for the output voltage.
It is another object of the present invention to provide a true root-mean square ac measuring device that provides amplitude correction for the output voltage.
Yet another object of the present invention is to provide a true root-mean square ac measuring device that is free of electromagnetic interference.
It is another object of the present invention to provide a rapid method of taking true root-mean square ac measurements, especially at high frequencies.
An opto-electric device for measuring the root mean square value of an alternating current voltage comprises: a) an electric field-to-light-to-voltage converter having 1) a light source; 2) an electro-optic material: (a) receiving light from the light source; (b) modulating said light; and (c) providing a modulated light output; 3) an electric field applied to the electro-optic crystal to modulate the light from the light source to produce the modulated light output; b) an optical receiver for receiving and converting the modulated output light from the electro-optic material to a first voltage that is proportional to a square of the electric field applied to the electro-optic material; c) an averager circuit receiving the first voltage and providing a second voltage that is proportional to the average of said square of said electric field over a period of time; and d) an inverse ratiometric circuit receiving the second voltage from the averager circuit and returning a third voltage that is an inverse voltage of the second voltage to the electric field-to-light-to-voltage converter to produce an output voltage that is the root mean square voltage of the applied electric field. The device uses a Mach-Zehnder interferometer operating as a square law device and features a housing for maintaining the interferometer at constant temperature using a temperature control unit. A nulling circuit is provided to maintain the interferometer at its null operating point as are calibration circuits to correct for voltage amplitude and frequency changes.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail and illustrated in the accompanying drawings. It is contemplated that variations in procedures, structural features and arrangement of parts may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.